This invention relates to an injection stretch blow molding method for molding a hollow molded product, such as a bottle, having a thin body portion by stretch blowing a preform, which has been injection molded, immediately after mold-release.
In stretch blow molding, there is the cold parison method, in which a preform having been injection molded is stocked, and heated and stretch blown later on by a stretch blowing apparatus; and the hot parison method in which an injection apparatus and a stretch blowing apparatus are provided in the same molding apparatus, and in which a preform having been injection molded is stretch blown while it possesses heat therein.
In both molding methods, the thickness and length of a stretched portion of the preform is set according to the volume and weight, and thickness of a body portion of a hollow molded product, such as a bottle or a wide-mouthed container, which becomes the final product. Further, according to a biaxial orientation caused by stretching in the longitudinal direction and thee radial direction, properties such as transparency and strength are enhanced. Therefore, the stretching ratio is set at as high a ratio as possible to an extent in that it is possible to obtain a thickness which maintains the functions as a hollow molded product.
The design of a preform which has conventionally been implemented is such that the stretching ratio in the longitudinal direction is set to be as large as possible by setting the thickness of a stretched portion of a preform to be thick, and, on the other hand, setting the length thereof short. However, such a preform with a thick thickness and a shortlength in the longitudinal direction has a considerable influence on the molding cycle or the molding state.
The more the thickness is increased, more time is required for cooling during preform molding in the hot parison method; and more time for preform heating is required in the old parison method. Thus, reduction in molding cycle time is impeded.
In a preform which is mold-released in a state maintaining high temperature for reducing the molding cycle, the accumulated amount of heat possessed in the inner portion thereof is proportional to the thickness. Accordingly, in a case where the thickness is thickened and the length is set shorter than before in order for the stretching ratio to be about 8 to 9 times to earn longitudinal stretching ratio, it is difficult to drop the inner temperature near glass transition temperature (Tg) by means of a cooling action according to airblow and increase in surface area caused by stretching. Therefore, a bias in thickness occurs and a satisfactory product cannot be obtained, even when the molding material is polyethylene terephthalate (PET) which-has a self-stretching characteristic.
Such a bias in thickness can be prevented by setting the longitudinal stretching ratio to be larger than before, and by dropping the preform temperature, in the final molding step by stretch molding, near glass transition temperature (Tg). However, even though such a method is applicable to molding a super-thin hollow molding product having a thickness of 0.15 mm or below with a high stretching ratio as disclosed in Japanese. Patent Laid-open Publication No. 9-94872, it is extremely difficult to apply this to a bottle having a usual geometry.
This invention has been contrived to solve the above-described problems, and an object is to provide a novel method which can stretch blow mold not only, needless to say, a bottle with a usual thickness, but also a hollow molded product with a usual geometry in which the thickness of its body portion is below 0.2 mm, and can reduce the molding cycle time, without forming the thickness of a stretched portion of the preform to be thick, even in the hot parison method which uses a preform having been mold-released in a state maintaining high temperature.
In order to achieve the above-described object, the present invention provides an injection stretch blow molding method characterized in that, in molding a hollow molded product having a thin body portion, such as a bottle, by molding a required preform by rapidly cooling molten resin which has been injected and filled into an injection mold, holding the preform by its neck portion with a neck mold, mold-releasing in a high temperature state in which the shape of the preform is maintained by a skin layer formed on the outer surface of the preform, and stretch blowing the preform laterally and longitudinally within a blow mold, among the thickness and length of a stretched portion of the preform which are set according to the volume and weight and body portion thickness of the hollow molded product, the thickness is set to be thinner than the usually set thickness and the length is set to be long inversely proportional to the thickness setting; and the preform is mold-released at a higher temperature than usual and is stretch-blow molded before its outer surface temperature reaches its peak.